Law enforcement officials arrested 141 people inNorth Dakota afterpolice surrounded protesters, deploying pepper spray and armored vehicles in order to clear hundreds of Native American activists and supporters from land owned by an oil pipeline company.

The move marked the beginning of an aggressive new phase in ongoing police efforts to thwart a months-long demonstration by hundreds of members of more than 90 Native American tribes to prevent the construction of the controversial Dakota Access pipeline, which they say would threaten the regional water supply and destroy sacred sites.

Clashes between Morton County law enforcement and protesters escalated on Thursday during a tense all-day standoff, as police pushed protesters off the private land where the pipeline is slated for construction, forcing activists to retreat back to the camps that have sprung up since the protest began in April.

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Activists vowed to continue fighting the project after the arrests. More activists were in custody but had yet to be processed, Morton County sheriff Kyle Kirchmeier said in a press conference.

Officials claimed that activists set pipeline equipment on fire, and erected illegal and dangerous roadblocks, forcing police to close in on the demonstration, deploy pepper spray and arrest activists who failed to comply with orders.

However, Rose Stiffarm, a cinematographer and member of six Native American nations including the Chippewa Cree, told the Guardian that the police response on Thursday was unnecessarily harsh. “The government is attacking us for protesting, for protecting the water.”

She said police were deploying teargas – something Kirchmeier denied – and said they were “targeting press”.

Dean Dedman Jr, a member of the Standing Rock Hunkpapa tribe from South Dakota, told the Guardian: “Everybody is still standing strong. We are still holding the lands.”

Dedman, who has been camped out for months, frequently shooting footage of the protests with his drone, said protesters would not back down even in the face of hundreds of armed police officers. “We’re all just trying to keep the prayer and keep the singing.”

Police have made more than 260 arrests since the demonstrations ramped up in August, and prosecutors have filed a range of charges, including criminal trespassing, participating in a riot and resisting arrest.

The first round of mass arrests came after a local judge rejected controversial riot charges against Amy Goodman, a well-known broadcast journalist and host of Democracy Now! who earned widespread support from free speech advocates after authorities issued a warrant for her arrest.

The Standing Rock tribe and a diverse group of Native American groups have been fighting to block the $3.7bn oil pipeline, which is operated by Texas-based Energy Transfer Partners and on track to transport fracked crude from the Bakken oil field in North Dakota to a refinery near Chicago.

The protests began in April when members of the Standing Rock Lakota and other nations took to horseback to ride to the river’s edge and set up a “spiritual camp”, named Sacred Stone, where activists returned on Thursday.

Cecily Fong, spokeswoman for the state department of emergency services, told the Guardian that the pipeline operator would be in charge of securing the site once law enforcement successfully removed activists. “Our intent from the beginning here is that no one gets seriously hurt. We’ve shown a lot of patience and discretion.”

The security response of the pipeline company, however, has proven to be controversial. On Wednesday, the Morton County sheriff’s office revealed that some private guards were not properly licensed when they deployed dogs on unarmed activists.

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In a hastily planned press briefing, Kirchmeier told reporters that police were planning to remain in place “as long as it takes” to keep protesters off the pipeline’s land. Fong also claimed that protesters set fire to multiple pipeline excavators.

Activists have also tried to fight the project in court, but in September, a judge denied an attempt by tribal leaders to challenge the legitimacy of the pipe’s construction permits. The federal government, however, announced last month that it would reassess initial approvals and delay permits.

Earlier in October, climate activists disrupted oil pipelines in North Dakota, Montana, Minnesota and Washington state, with protest group Climate Direct Action saying the move was in support of those opposing the Dakota Access pipeline.

Major protests also occurred in recent years over the proposed Keystone XL pipeline, which aimed to carry tar sands oil from Alberta to Nebraska and also pick up Bakken shale oil in Montana. Over 200 protesters were arrested in 2014 after strapping themselves to the White House fence. Another 50 were detained in 2013, during a rally led by climate scientist James Hansen, actor Daryl Hannah and environmental advocate Robert Kennedy Jr.

However, after seven years of fierce debate, Barack Obama rejected the pipeline in November 2015, saying: “If we’re going to prevent large parts of this Earth from becoming not only inhospitable but uninhabitable in our lifetimes, we’re going to have to keep some fossil fuels in the ground.”

The defeat of Keystone XL was a major coup for climate change campaigners, who are now battling to stop another proposed pipeline from TransCanada, the company behind Keystone XL. Called Energy East, it would take tar sand oils across Canada to the Atlantic, where it would be shipped to the Gulf of Mexico.

The North Dakota oil boom began in 2006 with the discovery of the Parshall oil field in the Bakken shale and the use of new fracking and horizontal drilling technology, which could force oil from formations that could not be previously tapped. The Bakken formation is the biggest oil producer in the US, along with Texas’s Eagle Ford, but production has peaked.